It is widely known that when an electric motor is driven by an inverter, a surge voltage is generated by high-speed switching in the inverter to affect insulation of a motor winding. Such a surge voltage is called “inverter surge” and may sometimes reach double or more of the rated voltage. When the inverter surge is applied to the motor windings, a partial discharge may occur inside or outside the windings. Such a partial discharge may cause a degradation of a coating of an enamel wire constituting the winding. The degradation of the coating may lead to an insulation breakdown. Thus, there is desired an insulation design of the electric motor that prevents the partial discharge from occurring even when the inverter surge is applied to the windings.
Conventionally, insulation performance of the motor windings is evaluated based on partial discharge characteristics obtained by application of a sinusoidal alternate voltage, especially, a partial discharge inception voltage. However, a potential distribution in the winding differs between when the sinusoidal alternate voltage is applied and when the surge voltage is applied. Therefore, the insulation performance of the inverter-driven electric motor is evaluated desirably by application of an impulse voltage simulating the surge voltage. Further, in partial discharge measurement conducted by application of the impulse voltage, voltage changes steeply at rising time (at application start time) of the impulse voltage, so that there is required a different approach from the partial discharge measurement conducted by application of the sinusoidal alternate voltage.
A guideline of a partial discharge measurement method that applies a repeated impulse voltage is internationally regulated. The guideline defines that an impulse voltage applied when the number of impulses at which a partial discharge is generated with respect to the number of all the applied impulse voltages reaches a specified frequency or more is regarded as a partial discharge inception voltage, which is called a partial discharge inception voltage under the repeated impulse voltage application. Further, a voltage is dropped after generation of the partial discharge by impulses equal to or more than a specified frequency, and the number of impulses at which the partial discharge is generated becomes less than the specified frequency. An impulse voltage at this time is regarded as a partial discharge extinction voltage, which is called a partial discharge extinction voltage under the repeated impulse voltage application. By measuring the partial discharge inception voltage under the repeated impulse voltage application or partial discharge extinction voltage under the repeated impulse voltage application, insulation performance of an inverter-driven electric motor is evaluated.
The partial discharge inception voltage under the repeated impulse voltage application is measured, for example, as follows. A measurement control program disposed on an oscilloscope instructs an impulse power source to output a predetermined voltage and takes in an applied voltage signal measured by a high-voltage probe and an output signal of a current transformer (CT) as a partial discharge sensor. The measurement control program instructs the impulse power source to output a voltage V1 10 times and then takes in applied voltages and signal data output from the CT for 10 times. When detecting an impulse at which the CT output signal exceeds a discharge determination specified value, the program determines that a partial discharge has been generated by that impulse.
In a case where the number of partial discharge generating impulses does not reach a predetermined number when the applied voltage is V1, the measurement control program instructs the impulse power source to output a higher voltage. Every time the measurement control program instructs the impulse power source to output a voltage V2, voltage V3, and voltage V4, it calculates the number of impulses at which the partial discharge is generated. For example, when the number of partial discharge generating impulses reaches a predetermined number, e.g., five at a voltage of V4, the voltage value V4 is regarded as the partial discharge inception voltage under the repeated impulse voltage application.
The voltage gradually drops from the voltage of V4, and the number of partial discharge generating impulses becomes less than a predetermined number. A voltage at this time is regarded as the partial discharge extinction voltage.